High power high efficiency cladding pumping fiber laser

ABSTRACT

In the present invention, methods and apparatus for making efficient cladding pumping fiber lasers is disclosed. In particular, new fiber cladding geometry and new method of coupling pumping laser into an optical fiber are disclosed. Both aspects of the present invention will facilitate the realization of high-efficiency and high-power fiber lasers. In the present invention, cladding boundary geometry structures that can prevent the formation of local modes are disclosed. Besides the cladding geometry, methods for efficient coupling of diode lasers into a laser fiber for high power injection are also disclosed. Essentially, the new methods make it possible to transmit diode laser beams for a long distance with substantially the same brightness. In other words, with the methods and apparatus disclosed, a laser beam from a laser diode array with collimating structures can be transmitted for a long distance while the beam spot dimension including beam divergence can be kept substantially unchanged even if the beam from said array is not substantially collimated. With the method taught in the present invention, laser diode array modules can be formed with laser diode arrays (LDA) with collimating structures and relay systems. Because of the brightness conservation nature of the modules, when a plurality of such modules (such as 3, 9 or more than 200 pieces) are combined, the beams from the arrays can be efficiently coupled into an optical fiber. Thus the pumping laser coupling method can be used for the construction of high-efficiency and high-power fiber lasers. Thus, the fiber lasers of this invention may comprise of a fiber laser core doped with active species, a symmetry-broken inner cladding or a multiple-imaging inner cladding surrounding said core, a plurality of brightness substantially-conserved laser diode array module for coupling the beam from said laser diode array module into said inner cladding.An apparatus for a high efficiency cladding pumping fiber laser component includes a laser fiber having a core doped with active species disposed along an axis. A symmetry- broken inner cladding surrounds said core for transmitting optical energy from the inner cladding to said core. The symmetry - broken inner cladding has a substantially rectilinear cross - section with respect to the axis with at least two pair opposing sides. At least one side of each pair of each opposing sides is disposed in nonparallel relationship to its respective opposed side whereby the inner cladding has a symmetry - broken cross - section shape that minimizes local modes in the inner cladding.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to cladding pumping fiber lasers, and inparticular relates to the fiber cladding geometry structure and pumpinglaser coupling method for construction of high-efficiency and high-powerfiber lasers.

2. Background Art

The development of diode pumped fiber lasers has been rather successfulrecently. The scaling of various physical effects has greatly benefitedthis development. Diode lasers can provide concentrated pumping energyand thus enhance the efficiency of fiber lasers. The long thin geometryof fiber also makes heat removal much easier than in bulk solid statelasers. In end-pumped fiber lasers, a large outer cladding is used incladding pumping. Pump light, often piped through fibers from pumplasers, enters the outer core, where it is confined so that it passesthrough the inner core, exciting the laser species. Stimulated emissionfrom the laser species remains in the inner core. By converting the lowbrightness beam from the pump diode bar into a tighter beam, pumping afiber laser can multiply brightness by a factor of more than 1000. Byusing such configuration, Polaroid, for example, reported a diodecladding pumping fiber laser reaching 35 W (M. Muedel et al CLEO '97,Postdeadline Paper CPD30, Baltimore, Md., 1997).

Currently, a typical fiber laser device includes a tens-of-meters doubleclad silica fiber with a small diameter and small NA core doped withactive species, centered within a much larger inner cladding, surroundedby a soft low index fluoropolymer providing an acceptance NA of 0.45 forpump radiation. Pumping laser beams from laser diodes are coupled intothe fiber inner cladding through the dichroic end mirror. (HR laser, HTpump). Among other things, the cladding pumping geometry and thecoupling efficiency of pumping laser diode array are the main factorsunder intense research. Proper geometry is essential for increasing theefficiency of cladding pumping. Good method of coupling will allow morepower to be injected into the fiber laser, which in turn also increasethe efficiency of a fiber laser.

There are many patents dealing with cladding pumping. U.S. Pat. Nos.5,533,163, 4,829,529, 4,815,079 disclose various claddingcross-sectional geometric shapes such as circle, rectangle, convexpolygon (triangle, rhombus, hexagon). These cladding boundary (CB)shapes, however, have certain obvious disadvantages. The maindisadvantage is the presence of local modes, and the pumping beamlocalized in such modes can not enter the core.

When skew ray is reflected on an arbitrary cylindrical surface, theprojection of incident ray and reflected ray on the principle crosssection are like the light ray reflected in this plane. Therefore, wecan use the behavior of light beams in the principal cross section ofcylindrical surface to determine local modes of fiber cladding.

The boundary shapes of fiber cladding geometry include circular,rectangle, right triangle, isosceles triangle, and rhombus. As acomparison with the current invention, the local modes in fiber claddingwith different boundary shape are summarized as follows.

FIG. 1 shows a schematic illustration of a light beam path in prior artcircular fiber cladding with a circular cladding boundary CB. Becausethe sag of ray keeps constant in multiple reflection, the light beamsLB₀ initial at the outer region can not reach central region throughmultiple reflection. Therefore, the center position is not a goodlocation for core A, and the core must be close to the boundary asposition B although center position is usually more preferred due to thestructures of fiber connections. FIG. 2 is a schematic illustrationshowing the local modes in prior art rectangular fiber cladding with aboundary CB. There are two types of local modes in the rectangularcladding. One is the light beam perpendicular to the boundary (LB₁, andLB₂), the other one is the light beam parallel to the line joining twocorners (LB₃). These light beams form different close cyclic loops inthe rectangular boundary. Besides these two fiber cladding geometryshapes, there are also right triangle, isosceles triangle and rhombus.For right triangle case, the light beams perpendicular to the hypotenuseform different close loops in the right triangle boundary as shown inFIG. 3 (LB₄ and LB₅). In isosceles triangle fiber cladding, there aretwo types of local modes as shown in FIG. 4. One is the light beamperpendicular to the leg (LB₆), the other type is the light beamparallel to the base (LB₇). Rhombus cladding behaves like two isoscelestriangles. The local modes in rhombus cladding are the same as inisosceles triangle as shown in FIG. 5.

If a fiber cladding has some local modes and the core is not in theregion of the local modes, the pumping light beam of local modes can notreach core and the pumping efficiency will decline.

In order to avoid the presence of local modes, sometimes bending in thefiber structure is suggested to provide perturbation in the modespropagating in the multi-mode cladding. However, the effect of bendingon perturbation is not clear, and can not be accurately predicted. Itwill be much more favorable to find new cladding geometry structures sothat local modes can not be generated, or at least the local modes arelimited near the core area and the pumping beam can easily enter thecore. In this way, the efficiency can be increased and the length offiber lasers can be reduced.

Currently, the leading company in fiber laser research and manufacturingis Polaroid. One fiber laser of Polaroid was reported to have highefficiency (about 65%), but this efficiency is the ratio of pumpinglaser power entered the optical fiber and the output power of the fiberlaser. Therefore, the efficiency of coupling or power injection is notconsidered. In this Polaroid fiber laser, three fiber-coupled SDL P6diodes are spatially combined and de-magnified into a rectangularcladding. The slop efficiency of the diode lasers is only 0.5 W/A, whilethe efficiency of a non-fiber coupled diode laser is much higher, 1.25W/A. Therefore the total electric efficiency of this Polaroid fiberlaser is not high. In addition, due to the limitation of the claddinggeometry, tens of meters of fiber must be used in this state-of-the-artsystem. Furthermore, since it is difficult to couple more beams into anoptical fiber with the coupling method used in the Polaroid systems, itis difficult to develop a fiber laser with even higher power, such as1000 W CW. It is therefore necessary to find new methods to couple highpower into optical fibers and improve the pumping efficiency.

A typical high-power laser diode array (LDA) has an a broad area lightemitting aperture (1 cm×1 μm) comprising light emitting elements whichare multiple spaced apart segments. In one typical commercial LDAproduct, for example, each segment has a width less than 200 μm, and maybe divided into 20 sub-segments. Each sub-segment has an aperture widthof 3-6 μm, and emits about 30 mW-60 mW. The effective aperture size inthe transverse direction perpendicular to the plane of laser activeregion (the fast axis) is about 1 μm. Typical fast axis divergence is30-40 degree and slow axis divergence is 10-15 degree. A typicalhigh-power LDA can deliver 20 W of laser power. Those more powerful candeliver 40 W or 60 W with this geometry. Because of the broad geometryof LDA, it has been always a challenge to couple or inject high power(such as 4000W) into a fiber cladding aperture (such as an aperture of200 μm×500 μm, NA 0.45).

In order to send more power into a cladding fiber, many efforts havebeen made to concentrate light from diode laser arrays. There are anumber of patents dealing with concentrating multiple emitter laserdiode beams, such as U.S. Pat. Nos. 5,802,092, 5,793,783, 5,790,310,5,594,752, 5,579,422, 5,568,577, 5,333,077, 5,185,758, 5,139,609, and4,428,647.

In U.S. Pat. No. 4,428,647, Spragne et al disclose systems in which eachlaser emitter of a diode laser array has its own lens mount adjacent toit in the space between the laser array and objective lens of thesystem. The purpose of the lens array is to change the angle ofdivergence of light beams leaving the emitting surface of the laserarray at the slow axis so that the light beam can be collectedefficiently by the objective lens. In U.S. Pat. No. 5,185,758 and in anearlier article (Optical Letters, Vol. 14, p.1087, 1989) Fan et aldescribe a method for scaling a pumped medium to higher power withmultiple light source. The output beam of each light source issubstantially collimated by respective collimating optics, and the beamsof sources are substantially parallel to each other after collimation.An optical system is provided to focus the collimated and parallelbeams. The methods described in U.S. Pat. No. 5,802,092, 5,793,783,5,790,310, 5,594,752, 5,579,422, 5,568,577, 5,333,077, and 5,139,609 aresimilar to the methods mentioned above. However, since lens arrays canonly collimate the beam from the diode array to a limited extent,obvious divergence still exists. Because of the beam divergence, laserdiode arrays must be close to an optical fiber so that the beam spot issmall enough to achieve effective coupling. When multiple laser diodearrays are combined, the dimension of beam spot on the fiber apertureplane becomes larger due to the increased distance between laser diodearrays and the fiber aperture. As a result, these methods can notefficiently combine the beams from a plurality of diode laser arrays into an optical fiber. For example, with these methods, it is impossibleto effectively couple the beams from 200 pieces of 20W diode laser barsinto a fiber to make a high-power fiber laser.

A need therefore exists in the art for a method to make high-efficiency,high-power fiber lasers, while high-efficiency cladding pumping geometryand effective LDA-fiber coupling are the most desired.

SUMMARY OF THE INVENTION

Accordingly, it is the principal object of the present invention toprovide a high-efficiency, high-power cladding pumping fiber laser.

It is also an object of the present invention to provide a method andapparatus with which a laser beam from a laser diode array withcollimating structures can be transmitted for a long distance while thebeam spot dimension can be kept substantially unchanged even if the beamfrom said array is not substantially collimated.

It is another object of the present invention to provide brightnessconserved optical pumping systems.

It is another object of the present invention to teach methods ofcoupling high laser power coupling into an optical fiber with highefficiency, such as a method of injecting at least 4000W power into0.5-0.2 mm²cladding and generating fiber laser output of 2500W.

It is another object of the present invention to provide novel claddinggeometry so that the length of laser fibers can be reduced.

It is still another object of the present invention to provide a fiberlaser with claddings that can avoid or minimize local modes.

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention, as embodied and broadly describedherein, a preferred apparatus of this invention may comprise a fiberlaser core doped with active species, a symmetry-broken inner claddingor a multiple-imaging inner cladding surrounding said core, a pluralityof brightness substantially-conserved laser diode array modules, and abrightness substantially-conserved optical system for coupling the beamfrom said laser diode array module into said inner cladding.

Additional objects, new features and advantages of the present inventionwill be set forth in part in the following description. Further scope ofapplicability of the present invention will become apparent from thedetail description of the invention provided hereinafter. It should beunderstood, however, that the detailed description of the invention andthe specific examples presented, while indicating preferred embodimentof present invention, are provided for illustration purposes only,because various changes and modifications within the scope and spirit ofthe present invention will become apparent to those of ordinary skill inthe art from the detail description of the invention that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings serve to explain the principles of theinvention, in which:

FIG. 1 shows a schematic illustration of light beam path in prior artcircular fiber cladding with a circular cladding boundary.

FIG. 2 is a schematic illustration showing the local modes in prior artrectangular fiber cladding.

FIG. 3 is a schematic drawing showing the local modes formed in claddingwith right triangle boundary.

FIG. 4 is a schematic drawing showing the local modes formed in claddingwith isosceles triangle boundary.

FIG. 5 is a schematic diagram showing the local modes formed in claddingwith rhombus boundary.

FIG. 6 is a schematic drawing showing the cross-section of asymmetry-broken circular cladding wherein a plane mirror boundary isadded to break the circular symmetry.

FIG. 7 is a schematic drawing showing the cross-section of asymmetry-broken circular cladding wherein a convex mirror boundary isadded to break the circular symmetry.

FIG. 8 is a schematic drawing showing a cross-section of asymmetry-broken circular cladding with a plane mirror boundary, whereinthe dimension of the mirror is larger than that in FIG. 6.

FIG. 9 is a schematic drawing showing the cross-section of asymmetry-broken circular cladding with a convex mirror boundary, whereinthe dimension of the mirror is larger than that in FIG. 7.

FIG. 10 is a schematic drawing showing a symmetry-broken rectangle-likeboundary, wherein the presence of small angles makes the surface nolonger parallel.

FIG. 11 is a schematic drawing showing a different symmetry-brokenrectangle-like boundary, wherein the presence of small angles makes thesurface no longer parallel.

FIG. 12 is a schematic drawing showing a multiple-imaging cladding forcladding pumping.

FIG. 13 is a schematic illustration showing a long light source TLpassing through the core by multiple-imaging cladding.

FIG. 14 is a schematic illustration showing the cross-section of arectangular-like multiple-imaging cladding, wherein two straight sidesof the rectangular-like shape in this cross-section are replaced withmultiple arcs.

FIG. 15 shows an illustration of LDAC1 in which a cylindrical lens CL isplaced in front of the laser bar so that the beam LB at the fast axis iscollimated.

FIG. 16 is a schematic illustration of LDAC2, wherein, besides acylindrical lens CL like in FIG. 15, a cylindrical lens array (CLA) isdisposed.

FIG. 17(a) is a schematic illustration of a fold prism PS along with thebeam path.

FIG. 17(b) shows how the three laser beams BM1, BM2, and BM3 are alignedby fold prisms wherein BM1 and BM3 after 4h reflection become the beamswith the same y coordinate and different x coordinate, and the beams arepiled up as shown in FIG. 17(c).

FIG. 18 is a schematic illustration for a combination of three LDAC's.

FIG. 19 is a schematic illustration for a combination of nine LDAC's.

FIG. 20(a) is a schematic illustration showing that without a relay,much larger dimension of the beams at the slow axis will be obtained.

FIG. 20(b) is a schematic illustration showing the improvement achievedwith a relay system.

FIG. 21(a) is a schematic illustration showing an example of a laserdiode array module used for multiple laser diode array pumping, with theside view in FIG. 21(b) showing that beam dimension can be kept the samefor a very long distance.

FIG. 21(c) is a schematic drawing showing a 4f telecentric relay with amagnification of 1:1, comprising cylindrical lens CL1 and CL2.

FIG. 21(d) is a schematic illustration showing a cylindrical telecentricrelay for a laser diode array module with a non-1:1 magnification.

FIG. 22 is a schematic illustration partially showing the combination ofmultiple laser diode array modules wherein the components are not inproportion, and many components and light beams are not shown for easyunderstanding.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, the methods and apparatus for making efficientcladding pumping fiber lasers are disclosed. In particular, new fibercladding geometry structures and new method of coupling pumping laserbeam into an optical fiber are disclosed. Both aspects of the presentinvention will facilitate the realization of high-efficiency andhigh-power fiber lasers. In the present invention, cladding boundarygeometry structures that can prevent the formation of local modes aredisclosed. The new cladding shapes can be made into large dimension ifdesired. Besides the cladding geometry, methods and apparatus forefficient coupling of diode lasers into a laser fiber for high powerinjection are also disclosed. Essentially, the new methods make itpossible to transmit diode laser beams for a long distance withsubstantially the same brightness. In other words, with the methods andapparatus disclosed, a laser beam from a laser diode array withcollimating structures can be transmitted for a long distance while thebeam spot dimension can be kept substantially unchanged even if the beamfrom said array is not substantially collimated. With the method taughtin the present invention, laser diode array modules can be formed withlaser diode arrays (LDA) with collimating structures and relay systems.Because of the brightness conservation nature of the modules, when aplurality of such modules (such as 3, 9, or more than 200 pieces) arecombined, the beams from the arrays can be efficiently coupled into anoptical fiber. Thus the pumping laser coupling method can be used forthe construction of high-efficiency and high-power fiber lasers. Thus,the fiber lasers of this invention may comprise a fiber laser core dopedwith active species, a symmetry-broken inner cladding or amultiple-imaging inner cladding surrounding said core, a plurality ofbrightness substantially-conserved laser diode array module for couplingthe beam from said laser diode array module into said inner cladding.

1. Fiber Cladding Geometry for Fiber Lasers

From the previous discussion of prior art, it can be seen that theboundary symmetry benefits the generation of local modes. Therefore, afavorite boundary for fiber cladding should be a symmetry-brokenboundary, and it is easy to break the symmetry of boundary withappropriate boundary perturbation.

Some examples of possible cladding geometry structures are discussedbelow. With this principle, it is apparent to the skilled in the artthat any inner cladding geometry structure with a symmetry-broken shapecan be used to reduce or eliminate the formation of local modes, andthis in turn will increase the efficiency of cladding fiber lasers.

(a). Symmetry-broken circular cladding

It is easy to break the symmetry of circular cylinder cladding. Examplesare shown in FIG. 6 and 8, by using a plane mirror, or in FIG. 7 and 9,by using a convex cylindrical mirror. Because of the abrupt change inthe direction of surface normal, all of whispering modes near boundarymay be destroyed. But some of the integral reflection loops may stillexist. FIG. 7 shows a three-reflection-loop LB₈. A larger cut of asshown in FIG. 8 and 9 can destroy these loops. By bending the fiber, theintegral reflection loop can also be destroyed. In these cases, thefiber core A can be positioned at the center of the cladding if desired.

(b) Symmetry-broken rectangular cladding

FIG. 10 and FIG. 11 show two cases of rectangle-like boundaries. Thepresence of small angles (θ1, θ2, θ3, θ4, 05, θ6, θ7) between theboundaries and the sides of a true rectangle (shown in dot line) makesthe surfaces no longer parallel to each other. In these configurations,the core A also can be located near the center.

Because of the area (volume) of core is much smaller than cladding,pumping beams in cladding must reflect multiple times before passingthrough the core. If the number of reflection is very large, the fiberlaser must be very long. The number of reflection may be larger than theratio of the dimension of cladding to core. One solution is to usemultiple-imaging claddings. It will then be possible to provide numbersof reflection substantially equal to the ratio between the claddingdimension and core dimension.

(c) Multiple-imaging cladding

FIGS. 12 and 13 show multiple-imaging cladding for claddings pumping. Itis the configurations that pile up multiple light source images at thecore. In FIG. 12, light source T1 forms an image at the core A by mirrorE1, light source T2 forms an image at T1 by mirror E2, light source T3form an image at T2 by mirror E3, and so on. The another half of lightbeam LB from T3 will be reflected back by mirror E4, and will enter thecore through multiple reflection by mirrors E3, E2, and E1. The mirrorsurfaces of E1, E2 and E3 may be circular cylinders or ellipticalcylinders. It can be seen that with the increase of light sources, thenumber of reflection will be increased in proportion with the increaseof the cladding dimension. This method will allow the arbitrary increaseof cladding dimension. Essentially, all of the light beams in thecladding will enter the small core through multiple imaging.

FIG. 13 shows a case of a long light source T_(L) passing through thecore by multiple-imaging cladding. Note that the light source and thecladding mirror are extended to the other direction of the core, so thecore is at the center of the cladding in this example. But the core doesnot have to be at the center to achieve the same results.

(d) Rectangular-like multiple-imaging cladding

Rectangular-like cladding described earlier can be modified to be amultiple-imaging cladding—rectangular-like multiple-imaging cladding.

Referring to FIG. 14, there shows a schematic illustration of thecross-section of a rectangular-like multiple-imaging cladding. The twostraight sides of the rectangular-like shape in this cross-section arereplaced with multiple arcs.

In this type cladding, it is preferred that arcs are circular-like, andthe centers of the circular arcs are on the opposite side of thecladding as shown in FIG. 14. Thus the center of arc P21-P22 is P12, thecenter of P11-P12 is P21 and so on. Therefore, P11-P12 is the image ofP12-P13, P13-P14, P14-P15, P15-P16, P16-P17, and P17-P18 by multiplereflection.

2. Lagrange Invariant of Laser Diode Arrays

It is well known that Lagrange invariant of an imaging optical system isthe product of the imaging beam solid angle and the image spot area. Theinvariant keeps constant from the radiation source to the optical systemand then to the image plane. Therefore, the brightness of light beamalso keeps constant throughout imaging process. A well-designed imagingoptical system is usually a brightness-conserved system.

Optical system for illumination may or may not be brightness-conserveddepending upon the system design. This can be seen in somespecifications of some 20 W diode laser arrays. For example, onecommercial product has an emitter area of 1 cm×1 μm and divergence angleof 10°×40°. The brightness is therefore 16 kW/(mm.rad)². Anotherfiber-coupled diode laser array starts with also a 20 W laser diodearray have a fiber output of 16W. This fiber-coupled diode laser arraywith a fiber core diameter of 0.6 mm and NA 0.37 only has a brightnessof 130 W/(mm.rad)². It can be seen that the design of the optical systemfor illumination must be very careful for coupling a diode laser beaminto a fiber, otherwise high-power diode laser beam can not beefficiently coupled into a fiber laser cladding due to the declinationof beam brightness.

Therefore, it is preferred to use high brightness LDA combining withbrightness conservation optical system for illuminating the fiber lasercladding. This will allow the injection of the highest power into afiber cladding of a given dimension.

LDA's with collimating structures (LDAC's) can have very differentLagrange invariant and therefore have very different brightnessdepending on the collimating structures used. When the power of an LDAis determined, it is preferred that the Lagrange invariant of LDAC is assmall as possible so that more power can be injected into a small area.Some laser diode arrays with collimating structures are shown belowalong with their Lagrange invariant. As examples, a 20 W laser diodearray is used in all these examples.

-   (a) LDAC1 has a laser bar with multiple emitters EM. The light    emitting aperture is 1 m×1 μm and the divergence angle is 10°×40°.    The Lagrange invariant of LDAC1 is 1.7×0.00073 (mm.rad). FIG. 15    shows an illustration of LDAC1 in which a cylindrical lens CL is    placed in front of the laser bar so that the beam LB at the fast    axis is collimated. If the focal length of CL is 1 mm, the aperture    of CL is 0.73 mm. If the cylindrical lens forms the image of the    emitter aperture of 1 μm at a distance of 730 mm, the image height    is also 0.73 mm. The laser beam leaves CL with an aperture of 1    cm×0.73 mm and a divergence angle of 10°0.001 rad, and has an    unchanged Lagrange invariant.-   (b) LDAC2 has the same light emitting aperture and divergence angle    as in LDAC1, but each emitter EM in this case is divided into 20    separate segments spaced apart. The width of each segment is 200    μm×1 m. Beam filling and collimating optics are used so that the    brightness of the individual segment is conserved. As shown in FIG.    16, besides a cylindrical lens CL like in FIG. 15, a cylindrical    lens array (CLA) is disposed in front of the emitter segments. In    contrast with LDAC1, the Lagrange invariant of LDAC2 is 0.68-0.00073    (mm.rad)².-   (c) LDAC3 is similar as LDAC2, but the segment width is changed from    200 μm to 150 μm. The Lagrange invariant of the LDA is 0.46×0.00073    (mm.rad)². With a structure just like that in FIG. 16, the beams are    collimated. A fill factor of 1 can be achieved, filling the aperture    and minimizing divergence angle θ. After CL and CLA, the beam from    LDAC3 has an aperture of 10×0.73 mm² and a divergence angle of    0.05×0.001 rad², and the Lagrange invariant is unchanged.-   (d) LDAC4 is a laser diode array with both fast axis and slow axis    collimators. In has no emitting aperture 1 cm×0.8 mm and a    divergence angle of 5°×3 mrad. The Lagrange invariant of LDAC4 is    0.87×0.0024 (mm.rad)².-   (e) LDAC5 is similar as LDAC1 but with two fold prisms. The prisms    align the laser beams and change Lagrange invariant of LDAC1 to    0.57×0.0022 (mm.rad)². FIG. 17(a) shows the schematic illustration    of the fold prism PS along with the beam path. The beam transmission    direction is changed due to the reflection in the fold prism. FIG.    17(b) shows how the three laser beams BM1, BM2, and BM3 are aligned.    These beams have the same x coordinate, but different y coordinate    and the beams propagate in the z direction. BM1 and BM3 after 4^(th)    reflection become the beams with the same y coordinate and different    x coordinate, and the beams are re-aligned and piled up as shown in    FIG. 17(c).-   (f) LDAC6 is the same as LDAC3 but with two fold prisms. Lagrange    invariant of LDAC6 is 0.15×0.0022 (mm.rad)². If the beams from these    different devices are focused by    an optics with NA 0.45, the spot dimensions are as the following:    LDAC1: 1.9×0.0008 mm², LDAC2: 0.75×0.0008 mm², LDAC3: 0.5×0.0008    mm², LDAC4: 0.96×0.0026 mm², LDAC5: 0.63×0.0023 mm², LDAC6:    0.17×0.0023 mm². Because the spot dimension in one direction is very    small, if we combine 200 pieces of the same LDAC's in this    direction, the dimension will be multiplied by 200. The spot    dimensions for these combined arrays will become 1.9×0.16,    0.75×0.16, 0.5×0.16, 0.96×0.54, 0.63×0.46, and 0.17×0.46 mm²,    respectively.

For a cladding pumping fiber laser, if the cladding dimension is largerthan the spot dimension shown above, the pumping diode laser power willbe able to be injected into the laser fiber efficiently. For the examplewith 200 pieces of LDAC's, this means 4000W of diode laser power can beinjected into the laser fiber. The LDAC's with folded prisms, disclosedabove in the present invention, apparently has advantages over othersdue to the small spot dimension. It should be noted, however, with theabove structures, none of the LDAC's can have a well collimated beamoutput at the slow axis.

3. Laser Diode Array Module

By analyzing the problems encountered in making high-efficiency,high-power fiber laser, a conclusion can be made that laser diode arrayswith collimating structures such as LDAC's discussed earlier can notprovide a substantially collimated beam. The spot dimension of the beamalways increases with the increase of distance between an LDAC and theaperture plane of an optical fiber, while the increase of this distanceis unavoidable when a plurality of LDAC's are combined. Therefore,although the fiber cladding can be highly efficient in allowing thepumping power to enter the fiber core of a fiber laser, the limit incoupling pumping laser beam into the fiber cladding still limits thetotal efficiency of a fiber laser. As a result, thus far there has notbeen a method to achieve a fiber laser with a high output power over,for example.

We discovered that the high efficiency of coupling laser into opticalfiber can be achieved by using a laser diode array module. A laser diodearray module is a system comprising at least one laser diode array withcollimating structures (LDAC's) and at least one optical relay system.An optical relay system is an optical system well defined in the art,with which the collimated or even a partially collimated beam from LDACcan be relayed to a more distant point. With the laser diode arraymodule, the beam from a LDAC can transmit for a long distance, while thebeam spot dimension including divergence angle after the transmissioncan be kept substantially the same as they leave the LDAC. Therefore,with said module, brightness conservation can be substantially achieved.It is obvious that both the LDAC's discussed above and other laser diodearrays with collimated structures can be used in said module. Theoptical relay system comprises at least one optical component such as alens. It is preferred that the optical relay system includes acylindrical lens. It is more preferred that the optical relay systeminclude a 4f cylindrical system. It is also more preferred that theoptical relay system is telecentric both at the objective space and theimage space. It is apparent to the skilled in the art that the relay canbe placed in any place in the optical path within the module, and it canalso be “mixed-in” with the collimating structures of LDAC's. Theadvantage of using laser diode array module will be obvious from theexamples below.

Referring to FIG. 18, there is shown in schematic illustration acombination of three LDAC's. In this example, each LDAC has acylindrical lens CL for collimating beam LB from LDA. The beam from theLDAC is collimated at the fast axis with the beam being divergent at theslow axis. The three beams are made parallel with small mirrors M, andthen focused by lens L to the common focal spot FS. If the aperture of alaser fiber coincides with FS, and the inner cladding dimension(including NA) is larger than the beam spot dimension, all of the laserbeams can be injected (or coupled) into the laser fiber. Because thelaser beams are divergent at the slow axis, the distance from LDAC's toL is made as short as possible so that the diameter of L can be small.If the NA of lens L is 0.45, the focal length of L will be equal to theLDAC beam dimension at the slow axis (for example, 10 mm for a typicalcommercial LDAC). If the LDAC's are like LDAC1, the focal spot dimensionat the slow axis is 1.7 mm. If the LDAC's are like LDAC4, the dimensionis 0.85. If the LDAC's have two folded prisms as in LDAC5 and LDAC6, thelaser beam dimension at the slow axis and the focal length of L is 3.3mm, and focal spot dimension will decrease to 0.56 and 0.28 mm,respectively. Because of the beam divergence at the fast axis is verysmall (typically 1 mrad), another dimension of focal spot is less than0.01 mm. In this case, the distance between LDAC and L can be small andrelay system is less important. But, with three LDAC's, only low powercan be achieved.

Referring to FIG. 19, there is shown in schematic illustration acombination of nine laser diode array modules. Each LDAC is collimatedin the fast axis. When there is no relay system, after the reflectionfrom the small mirrors M, the nine parallel laser beams are focused bylens L to a common focal spot FS. If the beam aperture at the fast axisis 0.8 mm, the combined beam aperture at AP will be 7.2 mm. Since thedistance d from LDAC to L can not be made small and may be even largerthan 50 mm, this will cause a too large dimension of the beams at theslow axis as shown in FIG. 20(a). It is necessary therefore to use arelay cylindrical lens CL3 with an LDAC to form a laser diode arraymodule as shown in FIG. 20(b). In this module, it is preferred that CL3is substantially close to the LDAC, and the focal length issubstantially equal to distance to aperture AP. If the NA at FS is 0.45,the spot dimension at slow axis is the same as the previous 3-LDA case,the spot dimension at the fast axis will be slightly larger than 0.01but still very small.

Because the beam dimension at the fast axis and at the slow axis aredifferent at the position AP, and because the position of the “object”is also different, lens L can be replaced with two cylindrical lenseshaving different focal lengths at different positions. However, in somespecial designs, single spherical lens still can be used as L. Inaddition, although NA of a beam at FS is limited as 0.45, the anglebetween the beam and the FS plane could exceed the value determined byNA since optical path is not telecentric. This is shown in FIG. 20(b).In other words, telecentric relay is more preferred.

From the above two examples, it can be seen that in order to combine thebeams from a plurality of LDAC's into a single aperture AP, the use oflaser diode array modules is necessary to achieve small focal spot. Withlaser diode array modules, beams can transmit in long distance withsubstantially the same brightness and keep being telecentric. Theadvantages of laser diode array modules therefore are obvious.

Referring to FIG. 21, there is shown in schematic illustration anexample of a laser diode array module used for multiple laser diodearray pumping. In this example, the laser diode array has multipleemitters EM. The laser beams from the emitters EM are collimated by thecylindrical lens CL at the fast axis. In general, the laser beam at thefast axis is diffraction limited. The beam dimension can be kept thesame for a very long distance (such as 800 mm within 0.8 mm), asschematically illustrated in FIG. 21(b) (side view). A cylindrical lensarray CLA is also disposed in the front of the laser diode array. Eachmicro-cylindrical lens of CLA collimates the beam from each emittersegment, respectively, at the slow axis, and allows each laser beam tofill each micro cylindrical lens aperture at the slow axis. Although thelaser beam is collimated by CLA, the divergence angle can still belarger than 5° at the slow axis. This means that, after the transmissionof 800 mm, the beam cross section will become 80 mm if the startingdimension is 10 mm. As an example, a telecentric optical system can beused as the large distance relay. If this telecentric relay is a 4fsystem with a magnification of 1:1, comprising cylindrical lens CL1 andCL2 as shown in FIG. 21(a) and 21(c), the divergent beam LB from CLA iscollimated by CL1 and focused by CL2 at aperture AP. In FIG. 21(c), f isthe focal length of CL1 and CL2. The laser beam dimension including thedivergence angle at AP is the same as it leaves from CLA.

Therefore, with laser diode array modules, laser beam can transmit along distance, such as 1m or longer, and still substantially preservethe brightness as from the LDAC, and concentrate at small area on AP.With laser diode array modules, it becomes not critical whether the beamfrom a LDAC is substantially collimated or not. The dimension of beamfrom a LDAC can be kept substantially the same after a long distancetransmission.

FIG. 21(d) shows a cylindrical telecentric relay for a laser diode arraymodule with a non-1:1 magnification, wherein f₁ and f₂ is the focallength of CL1 and CL2, respectively. Since the magnification is variable(f₂/f₂), the image dimension of the laser diode array at AP is variableat the slow axis, so that it is possible to make the laser beams havesubstantially the same dimensions at the slow axis and at the fast axis.

4. Combination of multiple laser diode array modules

We can combine multiple laser diode array modules as shown in FIG. 22,where LDA's are laser diode arrays with heat sink. Each LDAC composes anLDA, a cylindrical lens CL and a cylindrical lens array CLA. Assumingthe thickness of each LDAC is 10 mm, and the distance between each LDACis larger than 10 mm for thermal management. If there are 50-100 piecesof LDAC's at one side of the aperture AP, the largest distance from LDAto AP may be as much as 500-1000 mm. Using relays to form modules asmentioned above, the laser beams from all the laser diode array modulescan be projected onto AP. In the figure, M's are small mirrors thatdirect the laser beams form LDA to the aperture AP in parallel.

The LDAC's in FIG. 22 can be divided into several groups if preferred.Therefore, although the distances of each LDAC could be different fromthe other, each group can be made to have the same total distance to AP.In this case one set of large telecentric relay lens (CL1 and CL2, orCL1′ and CL2′) will be enough for each group. Thus in this example, onemodule comprises a plurality of LDAC's and one set of relay system.

With the scheme in FIG. 22, the dimension of laser beams at the fastaxis from the laser diode array modules can be kept substantiallyconstant over a long distance. The center to center distance at thefirst axis between beams is just equal to the beam dimension at the fastaxis. Therefore, if the dimension of each laser beam is 0.8 mm at thefast axis, the total dimension at the fast axis for laser beams from 200pieces of laser diode array modules will be about 160 mm at AP.

By using cylindrical lens to focus the beam at the fast axis into a beamof NA 0.45, the aperture of the lens is equal to the focal length. Whenf=160, the dimension of the focal spot at the fast axis will be 0.16 mmsince the divergence angle of the beam is 0.001 rad. If a 4f 1:1 relayis used in the laser diode module, the aperture in the other directionwill still be 10 mm. By focusing with cylindrical lens having f=10, thedimension of the focal spot at the slow axis will be 0.75 for LDAC2 and0.5 for LDAC3.

If the non 1:1 telecentric relay shown in FIG. 21(d) is used, theaperture at the slow axis can be 160 mm, and the divergence angledemagnified to 1/16. A spherical lens can then be used to focus thebeams into NA 0.45, and the focus spot dimension is the same as before.

With the components described above, such as laser diode array modulesand claddings, a high-efficiency fiber laser can be obtained, comprisingat least one laser diode array module, a laser fiber with its core dopedwith active species, and an imaging optical system, wherein said imagingoptical system is disposed between said module and the aperture of saidfiber and focuses the beam from said module onto the aperture of saidlaser fiber. Said imaging optical system can include lens and mirrors.For high-power fiber lasers, a plurality of said modules are used. Ifdesired, more than 200 modules can be used. It is preferred that thereis a symmetry-broken inner cladding or a multiple-imaging claddingsurrounding the core of said laser fiber. Said symmetry-broken claddingcan be a symmetry-broken circular cladding, a symmetry-brokenrectangular cladding, or other symmetry-broken cladding. Saidmultiple-imaging cladding can be a rectangular-like multiple-imagingcladding or other multiple-imaging cladding.

Said laser diode array module is a system comprising at least one laserdiode array, a collimating structure, and at least one optical relaysystem. It is preferred that said optical relay system is a 1:1 4fcylindrical relay, or include a non-1:1 cylindrical telecentric relay.Said collimating structure can include a plurality of fold prisms.

It is obvious to the skilled in the art, with the same principle, themethods described above can also be used for pumping solid-state laserrods. This can be easily realized if the laser fiber described above isreplaced with a laser rod. Of course, it is preferred if the roddimension substantially equal to or larger than the beam dimension fromthese systems. Therefore, a high-efficiency diode-pumped solid statelaser can be made, comprising at least one laser diode array module, alaser rod doped with active species such as rare-earth or transitionmetal ions like Nd, Yb, Er, Pr, and so on, and an imaging opticalsystem, wherein said imaging optical system is disposed between saidmodule and the aperture of said laser rod and focuses the beam from saidmodule onto the aperture of said laser rod. Said imaging optical systemcan include lens and mirrors. For high-power diode-pumped solid statelasers, a plurality of said modules are used. If desired, more than 200modules can be used. Said laser diode array module is a systemcomprising at least one laser diode array, a collimating structure, andat least one optical relay system. It is preferred that said opticalrelay system is a 1:1 4f cylindrical relay, or include a non-1:1cylindrical telecentric relay. Said collimating structure can include aplurality of fold prisms.

These methods and systems described above can also be used fortransmission of power from laser diode arrays through an optical fiber.High power can be transmitted because the method disclosed aboveallowing high efficiency in coupling laser power from a plurality oflaser diode arrays into an optical fiber. The high power transmitted inthe optical fiber can then be used, for instance, for materialprocessing and medical surgery directly. Therefore, an apparatus forlaser beam transmission using an optical fiber can be made, comprisingan optical fiber for laser transmission, at least one laser diode arraymodule, and an imaging optical system, wherein said imaging opticalsystem is disposed between said module and the aperture of said fiberand focuses the beam from said module onto the aperture of said opticalfiber, and wherein said laser diode array module comprises at least onelaser diode array, a collimating structure, and an optical relay system.Said imaging optical system can include lens and mirrors. For high powertransmission, a plurality of said modules are used. It is preferred thatsaid optical relay system is a 1:1 4f cylindrical relay, or include anon-1:1 cylindrical telecentric relay. Said collimating structure caninclude a plurality of fold prisms.

The foregoing descriptions of embodiments of the invention have beenpresented for the purpose of illustration and description. It is notintended to limit the invention to the precise form disclosed, andobviously many modification and variation are possible in light of aboveteaching. For example, symmetry-broken polygon or other multiple-imagingconfigurations can be used as fiber inner cladding. Fold prisms can beused in cascade to further shrink the beam dimension at the slow axis.Multiple lenses can be used as the relay system at the slow axis for abrightness substantially-conserved laser diode array module. Multiplerelays can also be used in cascade. The embodiments were chosen anddescribed in order to best explain the principles of the invention andits practical applications to thereby enable others skilled in the artto best utilize the present invention. While specific values have beenused and listed in the foregoing embodiments for the sake of easy tounderstand, they do not set limit on the invention teaching, which isproperly described in the claims. The particular values andconfigurations discussed above can be varied and are cited merely toillustrate particular embodiments of the present invention and are notintended to limit the scope of the present invention.

1. A high-efficiency cladding pumping fiber laser apparatus comprising:a laser fiber having cladding with its having a core doped with at leastone active species and defining an aperture, at least one laser diodearray module, for emitting a laser beam with a beam spot dimension, animaging optical system, wherein said the imaging optical system isdisposed between said the laser diode array module and an the apertureof said the laser fiber and focuses the laser beam from said the laserdiode array module onto the aperture of said the laser fiber; andwherein said the laser diode array module comprises at least one laserdiode array with a the beam having fast and slow axis divergence, acollimating structure, and an optical relay system; and wherein said theimaging optical relay system has at least one optical element for beamrelay and is disposed between said the laser diode array module and saidthe laser fiber, whereby the laser beam from said the laser diode arraymodule is relayed to a more distant point with the beam spot dimensionbeing kept substantially unchanged.
 2. An The apparatus of claim 1,wherein said the laser fiber has a symmetry-broken inner claddingsurround the core of said the laser fiber.
 3. An The apparatus of claim2, wherein said the symmetry-broken cladding is symmetry-broken circularcladding.
 4. An The apparatus of claim 2, wherein said thesymmetry-broken cladding is symmetry-broken rectangular cladding.
 5. AnThe apparatus of claim 1, wherein said the laser fiber has amultiple-imaging cladding surrounding the core of said the laser fiber.6. An The apparatus of claim 5, wherein said the multiple-imagingcladding is rectangular-like multiple-imaging cladding.
 7. An Theapparatus of claim 1, wherein said the optical relay system is acylindrical lens.
 8. An The apparatus of claim 1, wherein said theoptical relay system is a 1:1 4f cylindrical relay.
 9. An The apparatusof claim 1, wherein said the optical relay system is a non-1:1cylindrical telecentric relay.
 10. An The apparatus of claim 1, whereinsaid the collimating structure includes a plurality of fold prisms. 11.A high-efficiency diode-pumped solid state laser apparatus comprising: alaser rod doped with active species defining an aperture, at least onelaser diode array module for emitting a beam with a beam spot dimension,an imaging optical system, wherein said the imaging optical system isdisposed between said the laser diode array module and an the apertureof said the laser rod and focuses the beam from said the laser diodearray module onto the aperture of said the laser rod; and wherein saidthe laser diode array module comprises at least one laser diode arraywith a the beam having fast and slow axis divergence, a collimatingstructure, and an optical relay system; and wherein said the opticalrelay has at least one optical element for beam relay and is disposedbetween said the laser diode array and said the laser rod, whereby thelaser beam from said the laser diode array module is relayed to a moredistant point with the beam spot dimension being kept substantiallyunchanged.
 12. An The apparatus of claim 11, wherein said the opticalrelay system is a cylindrical lens.
 13. An The apparatus of claim 11,wherein said the optical relay system is a 1:1 4f cylindrical relay. 14.An The apparatus of claim 11, wherein said the optical relay system is anon-1:1 cylindrical telecentric relay.
 15. An apparatus for laser beamtransmission comprising: an optical fiber for laser beam transmission,at least one laser diode array module for emitting a beam with a beamspot dimension, an imaging optical system, wherein said the imagingoptical system is disposed between said the laser diode array module andan the aperture of said the optical fiber and focuses the beam from saidthe module onto the aperture of said the optical fiber; and wherein saidthe laser diode array module comprises at least one laser diode arraywith a the beam having fast and slow axis divergence, a collimatingstructure, and an optical relay system; and wherein said the opticalrelay has at least one optical element for beam relay and is disposedbetween said the laser diode array and said the optical fiber, wherebythe laser beam from said the laser diode array is relayed to a moredistant point with the beam spot dimension being kept substantiallyunchanged.
 16. An The apparatus of claim 15, wherein said the opticalrelay system is a cylindrical lens.
 17. An The apparatus of claim 15,wherein said the optical relay system is a 1:1 4f cylindrical relay. 18.An The apparatus of claim 15, wherein said the optical relay system is anon-1:1 cylindrical telecentric relay.
 19. An apparatus for a highefficiency cladding pumping fiber laser component comprising: a laserfiber including a core doped with active species disposed along an axis;a symmetry-broken inner cladding surrounding said core for transmittingoptical energy from the inner cladding to said core; the symmetry-brokeninner cladding having a substantially rectilinear cross-section withrespect to the axis with at least two pair opposing sides; and, at leastone side of each pair of each opposing sides disposed in nonparallelrelationship to its respective opposed side whereby the inner claddinghas a symmetry-broken cross-section shape that minimizes local modes inthe inner cladding.
 20. The apparatus for a high efficiency claddingpumping fiber laser in accordance with claim 19 wherein: each of theopposing sides are disposed in nonparallel relationship with respect toa rectangle.
 21. The apparatus for a high efficiency cladding pumpingfiber laser in accordance with claim 19 wherein: the at least one sidedisposed in nonparallel relationship includes a first segment disposedin a first nonparallel relationship and a second segment disposed in asecond nonparallel relationship.
 22. The apparatus for a high efficiencycladding pumping fiber laser in accordance with claim 19 including: anouter cladding disposed adjacent the inner cladding.
 23. A highefficiency cladding pumping fiber laser component comprising: a laserfiber including a core doped with active species disposed along an axis;a symmetry-broken inner cladding surrounding said core for transmittingoptical energy from the inner cladding to said core; at least one lightpath disposed along the symmetry-broken inner cladding; thesymmetry-broken inner cladding having a cross-section shape thatincludes multiple imaging cladding surrounding the core between thelight path and the laser fiber whereby the inner cladding has asymmetry-broken cross-section shape that minimizes local modes in theinner cladding.
 24. The high efficiency cladding pumping fiber lasercomponent of claim 23, wherein said multiple-imaging cladding isrectangular-like multiple-imaging cladding.
 25. The high efficiencycladding pumping fiber laser component of claim 23, further including:an outer cladding disposed adjacent the inner cladding.